The present invention relates to watthour meters for measuring AC electrical energy consumption by a load. More particularly, the invention relates to improvements in sensing line voltage in solid-state watthour meters.
Electronic or solid-state watthour meters have been known since the early 1970's. In meters of this type, the load current and line voltage are sensed and corresponding current and voltage signals are input into an integrated circuit chip. The current and voltage signals are multiplied within the measurement chip to produce an output signal corresponding to the AC electrical energy consumption by the load. This output signal is supplied to a register that converts this signal to a readable display. In more recent years, solid-state registers have become more sophisticated to include custom integrated circuits to perform usage and load profile calculations.
In the watthour meters of the prior art, voltage and current sensing have been performed using voltage or current transformers. These transformers reduce the voltage and current from the AC source to levels compatible with the solid-state components of the meter. The transformers also provide electrical isolation of the meter from the AC source to minimize the risk of accidental electrical shock. However, transformers, and particularly voltage transformers, are bulky and expensive. Moreover, transformer losses and phase errors reduce accuracy and frequently require additional circuitry to compensate for these errors.
It is, therefore, an object of the present invention to provide a watthour meter with means to sense the line voltage across an AC power source that is highly accurate, but at a lower cost than the voltage transformers of the prior art. It is a further object to provide means to sense line voltage that presents a smaller physical package than a voltage transformer. Another object is to provide means to electrically isolate the user accessible components of this watthour meter from the voltage and current of the AC power source. Yet another object is to provide novel means to feed electrical power, derived from the AC power source, to the user accessible components, while preserving the electrical isolation required for safety. Other objects of the invention will be made apparent from the following disclosure and accompanying figures.